cmd/go: clarify that -coverpkg uses import paths

This change amends the long-form help output for 'go help build' and
'go help testflag' to specify that the '-coverpkg' flag operates
explicitly on import paths as well as package names. Import paths are
fundamental for precise specification of packages versus unqualified
package names, and the naming of the flag '-coverpkg' and its original
documentation leads a user to assume that it only operates on the
simple, unqualified package name form. The situation warrants
clarification.

Fixes golang#69653

src/cmd/go/internal/help/helpdoc.go

@@ -64,6 +64,12 @@ Go library.
 - "cmd" expands to the Go repository's commands and their
 internal libraries.
 
+Package names that correspond to complete import paths found in the standard
+library can be used as patterns without further qualification. For instance,
+"fmt" refers to the standard library's package fmt, but "http" alone for
+package http could not be used to refer to import path "net/http". Instead,
+the complete pattern "net/http" must be used.
+
 Import paths beginning with "cmd/" only match source code in
 the Go repository.
 
@@ -93,7 +99,10 @@ By convention, this is arranged by starting each path with a
 unique prefix that belongs to you. For example, paths used
 internally at Google all begin with 'google', and paths
 denoting remote repositories begin with the path to the code,
-such as 'github.com/user/repo'.
+such as 'github.com/user/repo'. Package patterns should include this prefix.
+For instance, a package called 'http' residing under 'github.com/user/repo',
+would be addressed with the fully-qualified pattern:
+'github.com/user/repo/http'.
 
 Packages in a program need not have unique package names,
 but there are two reserved package names with special meaning.

src/cmd/go/internal/test/test.go

@@ -235,10 +235,10 @@ control the execution of any test:
 	    Sets -cover.
 
 	-coverpkg pattern1,pattern2,pattern3
-	    Apply coverage analysis in each test to packages matching the patterns.
-	    The default is for each test to analyze only the package being tested.
-	    See 'go help packages' for a description of package patterns.
-	    Sets -cover.
+	    Apply coverage analysis in each test to packages whose import paths
+	    match the patterns. The default is for each test to analyze only
+	    the package being tested. See 'go help packages' for a description
+	    of package patterns. Sets -cover.
 
 	-cpu 1,2,4
 	    Specify a list of GOMAXPROCS values for which the tests, benchmarks or

src/cmd/go/internal/work/build.go

@@ -104,10 +104,11 @@ and test commands:
 		Sets -cover.
 	-coverpkg pattern1,pattern2,pattern3
 		For a build that targets package 'main' (e.g. building a Go
-		executable), apply coverage analysis to each package matching
-		the patterns. The default is to apply coverage analysis to
-		packages in the main Go module. See 'go help packages' for a
-		description of package patterns.  Sets -cover.
+		executable), apply coverage analysis to each package whose
+		import path matches the patterns. The default is to apply
+		coverage analysis to packages in the main Go module. See
+		'go help packages' for a description of package patterns.
+		Sets -cover.
 	-v
 		print the names of packages as they are compiled.
 	-work